http://www.barefeats.com/mincd.html
There you go. Unless you play games, which the mini is basically incapable of anyway, the difference is minimal. And remember these are single app benchmarks, the larger memory will show better performance while switching between apps. Of your two choices I would pick 1GB + 256MB for general work.
But as long as you're opening the mini anyway, why bother to put in any less than 2x1GB... what I'd really do is either this or stick with the old memory.